System and method providing over current and over power protection for power converter

ABSTRACT

System and method for protecting a power converter. A system includes a threshold generator configured to generate a threshold signal, and a first comparator configured to receive the threshold signal and a first signal and to generate a comparison signal. The first signal is associated with an input current for a power converter. Additionally, the system includes a pulse-width-modulation generator configured to receive the comparison signal and generate a modulation signal in response to the comparison signal, and a switch configured to receive the modulation signal and adjust the input current for the power converter. The threshold signal is associated with a threshold magnitude as a function of time. The threshold magnitude increases with time at a first slope during a first period, and the threshold magnitude increases with time at a second slope during a second period. The first slope and the second slope are different.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 13/205,417, filedAug. 8, 2011, which is a continuation of U.S. Ser. No. 12/690,808, filedJan. 20, 2010, which is a continuation of U.S. Ser. No. 11/752,926,filed May. 24, 2007, which claims priority to CN 200710040379.9, filedApr. 28, 2007, and U.S. Ser. No. 11/752,926 is a continuation-in-part ofU.S. Ser. No. 11/213,657, filed Aug. 26, 2005, which claims priority toCN 200510029016.6, filed Aug. 18, 2005; all of the above applicationsare commonly assigned and incorporated by reference herein for allpurposes.

This application is related to U.S. patent application Ser. No.11/051,242, commonly assigned, incorporated by reference herein for allpurposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a control system and method forover-current protection and over-power protection. Merely by way ofexample, the invention has been applied to a power converter. But itwould be recognized that the invention has a much broader range ofapplicability.

Power converters are widely used for consumer electronics such asportable devices. The power converters can convert electric power fromone form to another form. As an example, the electric power istransformed from alternate current (AC) to direct current (DC), from DCto AC, from AC to AC, or from DC to DC. Additionally, the powerconverters can convert the electric power from one voltage level toanother voltage level.

The power converters include linear converters and switch-modeconverters. The switch-mode converters often use pulse-width-modulated(PWM) or pulse-frequency-modulated mechanisms. These mechanisms areusually implemented with a switch-mode controller including variousprotection components. These components can provide over-voltageprotection, over-temperature protection, over-current protection (OCP),and over-power protection (OPP). These protections can often prevent thepower converters and connected circuitries from suffering permanentdamage.

For example, a power converter includes a power switch and transformerwinding that is in series with the power switch. The current flowingthrough the power switch and transformer winding may be limited by anOCP system. If the OCP system is not effective, the current can reach alevel at which damage to the power switch is imminent due to excessivecurrent and voltage stress at switching or thermal run-away duringoperation. For example, this current level can be reached when theoutput short circuit or over loading occurs. Consequently, the rectifiercomponents on the transformer secondary side are subject to permanentdamage due to excessive voltage and current stress in many offlineflyback converters. Hence an effective OCP system is important for areliable switch mode converter.

FIG. 1 is a simplified conventional switch mode converter withover-current protection. A switch mode converter 100 includes an OCPcomparator 110, a PWM controller component 120, a gate driver 130, apower switch 140, resistors 150, 152, 154, and 156, and a primarywinding 160. For example, the OCP comparator 110, the PWM controllercomponent 120, and the gate driver 130 are parts of a chip 180 for PWMcontrol. When the current of the primary winding is greater than alimiting level, the PWM controller component 120 turns off the powerswitch 140 and shuts down the switch mode power converter 100.

For switch mode converter, a cycle-by-cycle or pulse-by-pulse controlmechanism is often used for OCP. For example, the cycle-by-cycle controlscheme limits the maximum current and thus the maximum power deliveredby the switch mode converter. This limitation on maximum power canprotect the power converter from thermal run-away. Some conventional OCPsystems use an adjustable OCP threshold value based on line inputvoltage, but the actual limitation on maximum current and thus maximumpower is not always constant over a wide range of line input voltage.Other conventional OCP systems use additional resistors 152 and 154 thatare external to the chip 180 and inserted between V_(in) and theresistor 150 as shown in FIG. 1. But the resistor 152 consumessignificant power, which often is undesirable for meeting stringentrequirements on low standby power. For example, the resistor 152 of 2 MΩcan dissipate about 70 mW with input AC voltage of 264 volts.

Hence it is highly desirable to improve techniques for over-currentprotection and over-power protection.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a control system and method forover-current protection and over-power protection. Merely by way ofexample, the invention has been applied to a power converter. But itwould be recognized that the invention has a much broader range ofapplicability.

According to one embodiment of the present invention, a system forprotecting a power converter is provided. The system includes a firstcomparator configured to receive a threshold signal and a first signaland to generate a comparison signal. The first signal is a sum of asecond signal and a third signal, and the third signal is associatedwith an input current for a power converter. Additionally, the systemincludes a pulse-width-modulation generator configured to receive thecomparison signal and generate a modulation signal in response to thecomparison signal, and a switch configured to receive the modulationsignal and control the input current for the power converter. Anamplitude for the first signal becomes larger if an amplitude for theinput voltage becomes larger. The second signal is generated byreceiving an input voltage for the power converter, converting thereceived input voltage to a fourth signal, and converting the fourthsignal to the second signal.

According to another embodiment, a system for protecting a powerconverter includes a first comparator configured to receive a firstsignal and a second signal and to generate a comparison signal. Thefirst signal is associated with an input current for a power converter.Additionally, the system includes a threshold generator configured toreceive at least a third signal and generate the second signal inresponse to at least the third signal. The third signal is associatedwith an input voltage for the power converter. Moreover, the systemincludes a pulse-width-modulation generator configured to receive thecomparison signal and generate a modulation signal in response to thecomparison signal, and a switch configured to receive the modulationsignal and control the input current for the power converter. Anamplitude for the second signal becomes smaller if an amplitude for theinput voltage becomes larger, and the third signal is generated byreceiving an input voltage for the power converter and converting thereceived input voltage to the third signal.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator configured to receive a thresholdsignal and a first signal and to generate a comparison signal. The firstsignal is equal to a second signal subtracted by a third signal, and thesecond signal is associated with an input current for a power converter.Additionally, the system includes a pulse-width-modulation generatorconfigured to receive the comparison signal and generate a modulationsignal in response to the comparison signal, and a switch configured toreceive the modulation signal and control the input current for thepower converter. Moreover, the system includes an oscillator coupled tothe pulse-width-modulation generator and configured to generate at leasta first control signal, a transconductor configured to receive the firstcontrol signal and generate a second control signal, and a currentsupplier configured to receive the second control signal and generate afirst current in response to the second control signal, the firstcurrent being associated with the third signal. An amplitude for thefirst current becomes smaller if an amplitude for the input voltagebecomes larger.

According to yet another embodiment, a method for protecting a powerconverter includes receiving an input voltage for a power converter,converting the received input voltage to a first signal, converting thefirst signal to a second signal, and generating a third signal based onat least information associated with the second signal. Additionally,the method includes receiving the third signal and a threshold signal.The third signal is a sum of the second signal and a fourth signal, andthe fourth signal is associated with an input current for the powerconverter. Moreover, the method includes generating a comparison signalbased on at least information associated with the third signal and thethreshold signal, and processing information associated with thecomparison signal. Also, the method includes generating a modulationsignal based on at least information associated with the comparisonsignal, and controlling the input current for the power converter inresponse to the modulation signal. An amplitude for the third signalbecomes larger if an amplitude for the input voltage becomes larger.

According to yet another embodiment, a method for protecting a powerconverter includes receiving an input voltage for a power converter,converting the received input voltage to a first signal, processinginformation associated with the first signal, generating a second signalbased on at least information associated with the first signal, andreceiving the second signal and a third signal. The third signal isassociated with an input current for a power converter. Additionally,the method includes generating a comparison signal based on at leastinformation associated with the second signal and the third signal,processing information associated with the comparison signal, generatinga modulation signal based on at least information associated with thecomparison signal, and controlling the input current for the powerconverter in response to the modulation signal. An amplitude for thesecond signal becomes smaller if an amplitude for the input voltagebecomes larger.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first signal based on at leastinformation associated with an input current for a power converter,generating a second signal, the second signal being proportional to aramping current, and processing information associated with the firstsignal and the second signal. Additionally, the method includesgenerating a third signal equal to the first signal subtracted by thesecond signal, receiving the third signal and a threshold signal,generating a comparison signal based on at least information associatedwith the third signal and the threshold signal, processing informationassociated with the comparison signal, generating a modulation signalbased on at least information associated with the comparison signal, andcontrolling the input current for the power converter in response to themodulation signal. An amplitude for the ramping current corresponding toa predetermined value for the input current becomes smaller if anamplitude for the input voltage becomes larger.

Many benefits are achieved by way of the present invention overconventional techniques. For example, some embodiments can provide anexcellent compensation for the “delay to output” by easily adjusting anexternal resistor. For example, the adjustment of the external resistortakes into account converter components external to a chip for PWMcontrol. Certain embodiments allow a maximum current and a maximum powerthat are constant over a wide range of input voltage. Some embodimentsconsume a low standby power by sharing a resistor for a sensing systemwith a startup system and/or a brownout protection system. For example,the resistor is shared by a sensing system and a startup system. Certainembodiments provide an excellent compensation for the “delay to output”without sensing an input voltage. For example, the pin counts for a chipfor PWM control is limited. In another example, the maximum width of aPWM signal is used to represent the input voltage. Different inputvoltages result in different maximum widths for the PWM signal, and thedifferent maximum widths result in different effective thresholdsignals. Some embodiments provide an over-current protection that caneffectively protect a power converter from excessive power, thermal runaway, excessive current and/or voltage stress.

According to yet another embodiment, a system for protecting a powerconverter includes a threshold generator configured to generate athreshold signal, and a first comparator configured to receive thethreshold signal and a first signal and to generate a comparison signal.The first signal is associated with an input current for a powerconverter. Additionally, the system includes a pulse-width-modulationgenerator configured to receive the comparison signal and generate amodulation signal in response to the comparison signal, and a switchconfigured to receive the modulation signal and adjust the input currentfor the power converter. The threshold signal is associated with athreshold magnitude as a function of time. The threshold magnitudeincreases with time at a first slope during a first period, and thethreshold magnitude increases with time at a second slope during asecond period. The first slope and the second slope are different.

According to yet another embodiment, a system for protecting a powerconverter includes a threshold generator configured to generate a firstthreshold signal, and a first comparator configured to receive the firstthreshold signal and a first input signal and to generate a firstcomparison signal. The first input signal is associated with an inputcurrent for a power converter, and the first threshold signal isassociated with a first threshold magnitude as a function of time.Additionally, the system includes a second comparator configured toreceive a second threshold signal and the first input signal and togenerate a second comparison signal. The second threshold signal isassociated with a second threshold magnitude. Moreover, the systemincludes a logic component configured to receive the first comparisonsignal and the second comparison signal and generate an output signal.Also, the system includes a pulse-width-modulation generator configuredto receive the output signal and to generate a modulation signal inresponse to the output signal, and a switch configured to receive themodulation signal and adjust the input current for the power converter.The first threshold magnitude increases with time at a first slopeduring a first period, and the first threshold magnitude increases withtime at a second slope during at least a second period. The first slopeand the second slope are different.

According to yet another embodiment, a system for protecting a powerconverter includes a current generator configured to generate a firstcurrent flowing into the current generator, and a comparator configuredto receive a threshold signal and a first signal and to generate acomparison signal. The first signal is a sum of a second signal and athird signal, the second signal is associated with the first current,and the third signal is associated with an input current for a powerconverter. Additionally, the system includes a pulse-width-modulationgenerator configured to receive the comparison signal and generate amodulation signal in response to the comparison signal, and a switchconfigured to receive the modulation signal and adjust the input currentfor the power converter. The first current is associated with a currentmagnitude as a function of time. The current magnitude increases withtime at a first slope during a first period, and the threshold magnitudeincreases with time at a second slope during a second period. The firstslope and the second slope are different.

According to yet another embodiment, a method for protecting a powerconverter includes generating a threshold signal, and receiving thethreshold signal and a first signal. The first signal is associated withan input current for a power converter. Additionally, the methodincludes processing information associated with the threshold signal andthe first signal, generating a comparison signal based on at leastinformation associated with the threshold signal and the first signal,processing information associated with the comparison signal, generatinga modulation signal based on at least information associate with thecomparison signal, and adjusting the input current for the powerconverter in response to the modulation signal. The threshold signal isassociated with a threshold magnitude as a function of time. Thethreshold magnitude increases with time at a first slope during a firstperiod, and the threshold magnitude increases with time at a secondslope during a second period. The first slope and the second slope aredifferent.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first threshold signal. The firstthreshold signal is associated with a first threshold magnitude as afunction of time. Additionally, the method includes receiving the firstthreshold signal and a first input signal. The first input signal isassociated with an input current for a power converter. Moreover, themethod includes processing information associated with the firstthreshold signal and the first input signal, generating a firstcomparison signal based on at least information associated with thefirst threshold signal and the first input signal, and receiving asecond threshold signal and the first input signal. The second thresholdsignal is associated with a second threshold magnitude. Also, the methodincludes processing information associated with the second thresholdsignal and the first input signal, generating a second comparison signalbased on at least information associated with the second thresholdsignal and the first input signal, receiving the first comparison signaland the second comparison signal, and generating an output signal basedon at least information associated with the first comparison signal andthe second comparison signal. Additionally, the method includesprocessing information associated with the output signal, generating amodulation signal based on at least information associated with theoutput signal, and adjust the input current for the power converter inresponse to the modulation signal. The first threshold magnitudeincreases with time at a first slope during a first period, the firstthreshold magnitude increases with time at a second slope during atleast a second period, and the first slope and the second slope aredifferent.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first current flowing into a currentgenerator, and receiving a threshold signal and a first signal. Thefirst signal is a sum of a second signal and a third signal, the secondsignal is associated with the first current, and the third signal isassociated with an input current for a power converter. Additionally,the method includes processing information associated with the thresholdsignal and the first signal, generating a comparison signal based on atleast information associated with the threshold signal and the firstsignal, processing information associated with the comparison signal,generating a modulation signal based on at least information associatedwith the comparison signal, and adjusting the input current for thepower converter in response to the modulation signal. The first currentis associated with a current magnitude as a function of time. Thecurrent magnitude increases with time at a first slope during a firstperiod, and the threshold magnitude increases with time at a secondslope during a second period. The first slope and the second slope aredifferent.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and the accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified conventional switch mode converter withover-current protection;

FIG. 2 is a simplified diagram showing relationship between extracurrent ramping amplitude and line input voltage;

FIG. 3 is a simplified diagram showing relationship between currentthreshold and line input voltage;

FIG. 4 is a simplified diagram showing relationship between thresholdoffset and line input voltage;

FIG. 5 is a simplified control system with constant maximum currentaccording to an embodiment of the present invention;

FIG. 6 is a simplified control system with constant maximum currentaccording to an embodiment of the present invention;

FIG. 7 is a simplified control system with constant maximum currentaccording to another embodiment of the present invention;

FIG. 8 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention;

FIG. 9 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention;

FIG. 10 is a simplified diagram showing relationship between PWM signalmaximum width and input voltage according to an embodiment of thepresent invention;

FIG. 11 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention.

FIG. 12 shows simplified current profiles for primary winding in CCMmode and DCM mode;

FIG. 13 shows a simplified diagram for maximum energy delivered to loadat each cycle as a function of input line voltage;

FIG. 14 is a simplified diagram showing relationship between currentlimit and PWM pulse width according to an embodiment of the presentinvention;

FIGS. 15(A) and (B) are simplified diagrams showing relationship betweenover-current threshold voltage with pulse width and input line voltagerespectively according to an embodiment of the present invention;

FIG. 16 shows a simplified diagram for maximum energy delivered to loadat each cycle as a function of input line voltage according to certainembodiments of the present invention;

FIG. 17 is a simplified control system for over-current and over-powerprotection according to yet another embodiment of the present invention;

FIG. 18 is a simplified diagram showing the threshold generator and thecomparator in the control system for over-current and over-powerprotection according to an embodiment of the present invention;

FIG. 19 is a simplified control system for over-current and over-powerprotection according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a control system and method forover-current protection and over-power protection. Merely by way ofexample, the invention has been applied to a power converter. But itwould be recognized that the invention has a much broader range ofapplicability.

As shown in FIG. 1, the current limit is expressed as follows:

$\begin{matrix}{I_{Limit} = {{\frac{V_{in}}{L_{p}} \times t_{on}} = \frac{V_{th\_ oc}}{R_{s}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where I_(Limit) represents the current limit. For example, the currentlimit is the current threshold for triggering over-current protection.Additionally, V_(in), is the input line voltage at node 190, and V_(th)_(_) _(oc) is the voltage level at an input terminal 112 of the OCPcomparator 110. R_(s) is the resistance of the resistor 150, and L_(p)is the inductance of the primary winding 160. Moreover, t_(on)represents on time of the power switch 140 for each cycle. Accordingly,the maximum energy ε stored in the primary winding 160 is

$\begin{matrix}{ɛ = {{\frac{1}{2} \times L_{p} \times I_{Limit}^{2}} = {PT}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where T represents the clock period, and P represents the maximum power.So the maximum power P can be expressed as follows:

$\begin{matrix}{P = {\frac{L_{p} \times I_{Limit}^{2}}{2\; T} = \frac{V_{in} \times t_{on}^{2}}{2 \times L_{p} \times T}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

Therefore the power can be limited by controlling the current limitI_(Limit). But Equation 3 does not take into account the “delay tooutput” that includes the propagation delay through a current sense pathto the power switch 140. For example, the propagation delay includespropagation delays through the OCP comparator 110, the PWM controllercomponent 120, the gate driver 130, and the response delay of turningoff of the power switch 140. During the “delay to output,” the powerswitch 140 remains on, and the input current through the switch 140keeps ramping up despite the current has already reached the thresholdlevel of the OCP comparator 110. The extra current ramping amplitude,ΔI, due the “delay to output” is proportional to the line input voltageV_(in) as follows:

$\begin{matrix}{{\Delta\; I} = {\frac{V_{in}}{L_{p}} \times T_{delay}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

where T_(delay) represents the “delay to output.” FIG. 2 is a simplifieddiagram showing relationship between extra current ramping amplitude andline input voltage. As shown in FIG. 2, the actual maximum currentI_(PEAK1) that corresponds to higher V_(in) is larger than the actualmaximum current I_(PEAK2) that corresponds to lower V_(in). Accordingly,the actual maximum power is not constant over a wide range of line inputvoltage. Hence the actual maximum power is expressed as follows:

$\begin{matrix}{P = {\frac{L_{p} \times \left( {I_{Limit} + {\Delta\; I}} \right)^{2}}{2\; T} = \frac{V_{in} \times \left( {t_{on} + T_{delay}} \right)^{2}}{2 \times L_{p} \times T}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

For example, T_(delay) depends on internal delays, gate charges, andcircuitry related to the gate driver 130. In another example, for thepredetermined switch mode converter 100, T_(delay) is constant, andhence the actual maximum power depends on the line input voltage. Tocompensate for variations of the actual maximum power, the threshold forover-current protection should be adjusted based on the input linevoltage.

FIG. 3 is a simplified diagram showing relationship between currentthreshold and line input voltage. The line input voltage V_(in2) islower than the line input voltage V_(in1), and the current thresholdI_(th) _(_) _(oc) _(_) _(vin2) for V_(in2) is larger than I_(th) _(_)_(oc) _(_) _(vin1) for V_(in1) as shown in FIG. 3. The current thresholddecreases with increasing line input voltage V_(in). At the currentthreshold, the over-current protection is triggered. The resultingmaximum current I_(PEAK1) for higher V_(in) is the same as the resultingmaximum current I_(PEAK2) for lower V_(in).

For example, the current threshold has the following relationship withthe line input voltage:

$\begin{matrix}{I_{th\_ oc} \approx {{I_{th\_ oc}\left( V_{{in}\; 1} \right)} - {\frac{V_{in} - V_{{in}\; 1}}{L_{p}}T_{delay}}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

where I_(th) _(_) _(oc) is the current threshold, V_(in) is the lineinput voltage, L_(p) is the inductance of the primary winding, andT_(delay) is the “delay to output.” Additionally, I_(th) _(_)_(oc)(V_(in1)) is the current threshold that is predetermined for theinput line voltage V_(in1). For example, V_(in1) is the minimum lineinput voltage. In another example, the current is sensed that flowsthrough the power switch and the primary winding. If the sensed currentreaches I_(th) _(_) _(oc), the PWM controller component sends a signalto turn off the power switch. After “delay to output,” the power switchis turned off.

In Equation 6, the second term

$\frac{V_{in} - V_{{in}\; 1}}{L_{p}}T_{delay}$represents a threshold offset to compensate for the effects of “delay tooutput.” FIG. 4 is a simplified diagram showing relationship betweenthreshold offset and line input voltage. As shown in FIG. 4, the term

$\frac{T_{delay}}{L_{p}}$is the slop that depends on the “delay to output” and the inductance ofprimary winding. As shown in FIG. 4, the current threshold decreaseswith increasing line input voltage.

For certain applications, it is difficult to estimate the thresholdoffset on chip for PWM control in order to compensate for the “delay tooutput.” For example, T_(delay) depends on converter components that areinternal and as well as external to the chip. The external componentsmay include a power MOSFET. Different types of power MOSFETs can havedifferent gate charges, which in turn result in different “delays tooutput.” Also, the external components may include the primary winding.Different types of primary windings can have different inductancevalues. In another example, the gate driver on the chip is intentionallymade slow for longer T_(delay).

Certain embodiments of the present invention provide systems and methodsthat allow maximum currents that are constant over wide range of inputvoltage. For example, these systems and methods are implemented inswitch mode converters. In another example, the input voltage is theinput line voltage for power converters.

In some embodiments, the input voltage is sensed and used to control acurrent source. The current source outputs a current that is used togenerate an offset signal through an adjustable resistor. For example,the adjustable resistor is external to a chip for PWM control. Theoffset signal is superimposed on a current sensing signal, and thissuperimposition provides a threshold offset to a predetermined currentthreshold as shown in Equation 6.

These embodiments of the present invention include examples of FIGS. 5through 9. FIGS. 5-9 are simplified control systems with constantmaximum current according to certain embodiments of the presentinvention. These diagrams are merely examples, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

As shown in FIGS. 5-7, an input voltage V_(in) is sensed and used tocontrol a current source. The current source generates a current I__(vin) as follows:

$\begin{matrix}{I_{\_ vin} = {\alpha\frac{V_{in}}{R_{sv}}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

where α is a constant. The current I_ _(vin) is used to generate anoffset signal through an adjustable resistor R. For FIGS. 5 and 6, theoffset signal is superimposed to a current sensing signal. Accordingly,an input signal to a comparator is the summation of sensed signalI_(sense)×R_(s) and the offset signal I_ _(vin) ×R. If the input signalreaches the threshold signal V_(th) _(_) _(oc), a gate driver iscommanded to turn off a power switch. Accordingly, when the over-currentprotection is triggered,I _(sense) ×R _(s) +I_ _(vin) ×R=V _(th) _(_) _(oc)  (Equation 8A)

The effective threshold signal I_(th) _(_) _(oc) is

$\begin{matrix}{I_{th\_ oc} = \frac{I_{sense}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 9} \right) \\{{Therefore},{I_{th\_ oc} = {{\frac{V_{th\_ oc}}{R_{s}} - \frac{I_{\_ vin} \times R}{R_{s}}} = {\frac{V_{th\_ oc}}{R_{s}} - \frac{\alpha \times {Vin} \times R}{R_{s} \times R_{sv}}}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

From Equations 6 and 10, the following relationship can be derived:

$\begin{matrix}{{I_{th\_ oc}\left( V_{{in}\; 1} \right)} = {\frac{V_{th\_ oc}}{R_{s}} - {\frac{V_{{in}\; 1}}{L_{p}}T_{delay}}}} & \left( {{Equation}\mspace{14mu} 11} \right) \\{{{and}\mspace{14mu}\frac{V_{in}}{L_{p}}T_{delay}} = \frac{\alpha \times V_{in} \times R}{R_{s} \times R_{sv}}} & \left( {{Equation}\mspace{14mu} 12} \right) \\{{{According}\mspace{14mu} R} = {\frac{R_{s} \times R_{sv}}{\alpha\; L_{p}}T_{delay}}} & \left( {{Equation}\mspace{14mu} 13} \right)\end{matrix}$

Hence the effects of “delay to output” can be compensated by adjustingthe resistor R for given L_(p), R_(sv), and R_(s) in a switching modeconverter.

For FIG. 7, the offset signal is subtracted from the threshold signal togenerate an effective threshold signal V_(th) _(_) _(oc) _(_) ^(eff).The effective threshold signal is provided to a comparator. Anotherinput of the comparator receives the sensed signal I_(sense)×R_(s). Ifthe sensed signal reaches the effective threshold signal V_(th) _(_)_(oc) _(_) _(eff), a gate driver is commanded to turn off a powerswitch. Accordingly, when the over-current protection is triggered,I _(sense) ×R _(s) =V _(th) _(_) _(oc) _(_) _(eff) =V _(th) _(_) _(oc)−I_ _(vin) ×R  (Equation 8B)

Therefore, Equations 9-13 are still valid. The effects of “delay tooutput” can be compensated by adjusting the resistor R for given L_(p),R_(sv), and R, in a switching mode converter.

FIG. 5 is a simplified control system with constant maximum currentaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 500 includesresistors 510, 512, and 514, current suppliers 520 and 522, a startupsystem 552, a comparator 530, a pulse-width-modulation (PWM) generator540, a sensing system 550, a threshold generator 560, and a switch 570.Although the above has been shown using a selected group of componentsfor the system 500, there can be many alternatives, modifications, andvariations. For example, some of the components may be expanded and/orcombined. Other components may be inserted to those noted above. Forexample, the system 500 includes an oscillator 580, which sends a clocksignal and a ramping signal to the PWM generator 540. In anotherexample, the system 500 includes a primary winding 582 with aninductance value of L_(p). Depending upon the embodiment, thearrangement of components may be interchanged with others replaced. Forexample, the system 500 is used to regulate a power converter. Furtherdetails of these components are found throughout the presentspecification and more particularly below.

For example, an input voltage V_(in) at node 590 is sensed by thesensing system 550 through the resistor 512 of R_(sv). In oneembodiment, the resistor 512 has a resistance value ranging from severalhundred kilo-ohms to several mega-ohms. In another embodiment, thesensing system 550 sends a signal to the current supplier 520. Forexample, the current supplier 520 is a current source. The currentsupplier 520 generates a current I_ _(vin) flowing through the resistor510 of R and generating an offset signal. The offset signal issuperimposed to a current sensing signal. For example, the currentsensing signal is generated by the resistor 514 of R_(s). The summationof the offset signal and the current sensing signal is provided to aninput 532 of the comparator 530. For example, the summation is in thevoltage domain. At the comparator 530, the summation is compared with apredetermined threshold signal generated by the threshold generator 560.For example, the threshold generator 560 receives a current I_ _(vin1)and a reference voltage V_(ref). Based on the comparison, the comparator530 sends a signal to the PWM generator 540. For example, the PWMgenerator 540 includes a PWM comparator 542, a logic controller 544, anda gate driver 546. The logic controller receives the signal sent fromthe comparator 530. In another example, the PWM comparator 542 receivesthe clock signal and the ramping signal generated by the oscillator 580.The PWM generator 540 receives the signal from the comparator 530 andcontrols the switch 570 through the gate driver 546. Additionally, thesensing system 550 sends a signal to the current supplier 522 accordingto an embodiment of the present invention. For example, the currentsupplier 522 is a current source. The current supplier 522 generates thecurrent I_ _(vin1) received by the threshold generator 560. The effectsof “delay to output” can be compensated by adjusting the resistor R. Forexample, the system 500 allows a maximum current that is constant over awide range of the input voltage V_(in). In another example, the resistor510 is adjusted for given L_(p), R_(sv), and R_(s) in a switching modeconverter according to Equation 13. In yet another example, the startupsystem 552 is connected to the sensing system 550, and is used tocontrol powering up of a chip for PWM control.

FIG. 6 is a simplified control system with constant maximum currentaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 600 is an exampleof the system 500. As shown in FIG. 6, the input voltage V_(in) at thenode 590 is converted by the resistor 512 of R_(sv) into a current. Thecurrent is sensed by a transistor 610 of MP1 through a voltage VDD atnode 620. For example, the node 620 is connected to a capacitor 630 ofC1, which is charged by the input voltage V_(in) and used to start upthe chip for PWM control. In another example, the current I sensed bythe transistor 620 is

$\begin{matrix}{I = {\frac{V_{in} - {VDD}}{R_{sv}} \approx \frac{V_{in}}{R_{sv}}}} & \left( {{Equation}\mspace{14mu} 14} \right)\end{matrix}$

where VDD is negligible in comparison with V_(in). Therefore the sensedcurrent I is a function of the input voltage_(in). As shown in FIG. 6,the sensed current is mirrored by another transistor 612 of MP2 togenerate another current I1. For example, the current mirror includesboth the transistors 610 and 612. The current I1 is further mirrored bya transistor 614 of MN1 and a transistor 616 of MN2 to generate acurrent I2. The current I2 is mirrored by a transistor 618 of MP3 and atransistor 619 of MP4 to generate a current I_ _(vin) . For example, thecascade transistors are used to boost the output impedance of thecurrent mirrors. In another example, the current I_ _(vin) isproportional to the current I sensed by the transistor 620. The currentI_ _(vin) is used to generate the offset signal through the resistor 510of R. The offset signal is superimposed to the current sensing signal.The generated signal V_(cs) at node 622 isV _(cs) =I _(sense) ×R _(s) +I_ _(vin) ×R  (Equation 15)

As shown in FIG. 6, the signal V_(cs) is provided to an input 532 of thecomparator 530. At the comparator 530, the signal V_(cs) is comparedwith a predetermined threshold signal generated by the thresholdgenerator 560. In response, the comparator 530 sends a signal to the PWMgenerator 540, which controls the switch 570. Additionally, transistors640 and 642 generate the current I_ _(vin1) , which is received by thethreshold generator 560.

FIG. 7 is a simplified control system with constant maximum currentaccording to another embodiment of the present invention. This diagramis merely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 700 includesresistors 710, 712, and 714, a current supplier 720, a startup system752, a comparator 730, a pulse-width-modulation (PWM) generator 740, asensing system 750, a threshold generator 760, and a switch 770.Although the above has been shown using a selected group of componentsfor the system 700, there can be many alternatives, modifications, andvariations. For example, some of the components may be expanded and/orcombined. Other components may be inserted to those noted above. Forexample, the system 700 includes an oscillator 780, which sends a clocksignal and a ramping signal to the PWM generator 740. In anotherexample, the system 700 includes a primary winding 782 with aninductance value of L. In yet another embodiment, the system 700includes a signal generator 762 for providing a voltage referencesignal. Depending upon the embodiment, the arrangement of components maybe interchanged with others replaced. For example, the system 700 isused to regulate a power converter. Further details of these componentsare found throughout the present specification and more particularlybelow.

For example, an input voltage V_(in) at node 790 is sensed by thesensing system 750 through the resistor 712 of R_(sv). In oneembodiment, the resistor 712 has a resistance value ranging from severalhundred kilo-ohms to several mega-ohms. In another embodiment, thesensing system 750 receives a signal from the startup system 752, andsends a signal to the current supplier 720. For example, the startupsystem 752 is connected to the input voltage V_(in) through the resistor712, and is used to control powering up of a chip for PWM control. Inanother example, the current supplier is a current source. The currentsupplier generates a current I_ _(vin) flowing through the resistor 710of R and generating an offset signal. For example, the offset signal isan offset voltage. The offset signal is provided to the thresholdgenerator 760, which also receives a voltage reference signal from thesignal generator 762. The threshold generator 760 provides an effectivethreshold signal V_(th) _(_) _(oc) _(_) _(eff) to an input 734 of thecomparator 730. Additionally, a current sensing signal is received by aninput 732 of the comparator 730. For example, the current sensing signalis generated by the resistor 714 of R_(s). In another example, thecurrent sensing signal is in the voltage domain.

At the comparator 730, the current sensing signal is compared with theeffective threshold signal V_(th) _(_) _(oc) _(_) _(eff). Based on thecomparison, the comparator 730 sends a signal to the PWM generator 740.For example, the PWM generator 740 includes a PWM comparator 742, alogic controller 744, and a gate driver 746. The logic controllerreceives the signal sent from the comparator 730. In another example,the PWM comparator 742 receives the clock signal and the ramping signalgenerated by the oscillator 780. The PWM generator 740 receives thesignal from the comparator 730 and controls the switch 770 through thegate driver 746. The effects of “delay to output” can be compensated byadjusting the resistor R. For example, the system 700 allows a maximumcurrent that is constant over a wide range of the input voltage V. Inanother example, the resistor R is adjusted for given L_(p), R_(sv), andR_(s) in a switching mode converter according to Equation 13.

According to other embodiments of the present invention, an inputvoltage V_(in) is sensed and used to control a current source as shownin FIGS. 8 and 9. The current source generates a current I_ _(vin) asfollows:or I_ _(vin) =βV _(in)  (Equation 14)

where β is a constant. The current I_ _(vin) is used to generate anoffset signal through an adjustable resistor R. For FIG. 8, the offsetsignal is superimposed to a current sensing signal. Accordingly, aninput signal to a comparator is the summation of sensed signalI_(sense)×R and the offset signal I_ _(vin) ×R. If the input signalreaches the threshold signal V_(th) _(_) _(oc), a gate driver iscommanded to turn off a power switch. Accordingly, when the over-currentprotection is triggered,I _(sense) ×R _(s) +I_ _(vin) ×R=V _(th) _(_) _(oc)  (Equation 15A)

The effective threshold signal I_(th) _(_) _(oc) is

$\begin{matrix}{I_{th\_ oc} = \frac{I_{sense}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 16} \right) \\{{Therefore},{I_{th\_ oc} = {{\frac{V_{th\_ oc}}{R_{s}} - \frac{I_{\_ vin} \times R}{R_{s}}} = {\frac{V_{th\_ oc}}{R_{s}} - \frac{\beta \times V_{in} \times R}{R_{s}}}}}} & \left( {{Equation}\mspace{14mu} 17} \right)\end{matrix}$

From Equations 6 and 17, the following relationship can be derived:

$\begin{matrix}{{I_{th\_ oc}\left( V_{{in}\; 1} \right)} = {\frac{V_{th\_ oc}}{R_{s}} - {\frac{V_{{in}\; 1}}{L_{p}}T_{delay}}}} & \left( {{Equation}\mspace{14mu} 18} \right) \\{{{and}\mspace{14mu}\frac{V_{in}}{L_{p}}T_{delay}} = \frac{\beta \times V_{in} \times R}{R_{s}}} & \left( {{Equation}\mspace{14mu} 19} \right) \\{{{According}\mspace{14mu} R} = {\frac{R_{s}}{\beta\; L_{p}}T_{delay}}} & \left( {{Equation}\mspace{14mu} 20} \right)\end{matrix}$

Hence the effects of “delay to output” can be compensated by adjustingthe resistor R for given L_(p) and R_(s) in a switching mode converter.

For FIG. 9, the offset signal is subtracted from the threshold signal togenerate an effective threshold signal V_(th) _(_) _(oc) _(_) _(eff).The effective threshold signal is provided to a comparator. Anotherinput of the comparator receives the sensed signal I_(sense)×R_(s). Ifthe sensed signal reaches the effective threshold signal V_(th) _(_)_(oc) _(_) _(eff), a gate driver is commanded to turn off a powerswitch. Accordingly, when the over-current protection is triggered,I _(sense) ×R _(s) =V _(th) _(_) _(oc) _(_) _(eff) =V _(th) _(_) _(oc)−I_ _(vin) ×R  (Equation 15B)

Therefore, Equations 16-20 are still valid. The effects of “delay tooutput” can be compensated by adjusting the resistor R for given L_(p)and R_(s) in a switching mode converter.

FIG. 8 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 800 includesresistors 810, 811, 812, and 814, a comparator 830, apulse-width-modulation (PWM) generator 840, a sensing system 850, atransconductor 852, a threshold generator 860, and a switch 870.Although the above has been shown using a selected group of componentsfor the system 800, there can be many alternatives, modifications, andvariations. For example, some of the components may be expanded and/orcombined. Other components may be inserted to those noted above. Forexample, the system 800 includes an oscillator, which sends a clocksignal and a ramping signal to the PWM generator 840. In anotherexample, the system 800 includes a primary winding 882 with aninductance value of L_(p). In yet another example, the system 800includes a brownout protection system 854. Depending upon theembodiment, the arrangement of components may be interchanged withothers replaced. For example, the system 800 is used to regulate a powerconverter. Further details of these components are found throughout thepresent specification and more particularly below.

For example, an input voltage V_(in) at node 890 is received by avoltage divider to generate a voltage βV_(in). For example, the voltagedivider includes the resistors 811 and 812, which are external to thechip for PWM control. The voltage βV_(in) is received by the sensingsystem 850 to generate a voltage signal. The voltage signal is sent tothe transconductor 852, which generates a current I_ _(vin) flowingthrough the resistor 810 of R and generating an offset signal. Forexample, the transconductor 852 is a voltage-controlled current source.The offset signal is superimposed to a current sensing signal. Forexample, the current sensing signal is generated by the resistor 814 ofR_(s). The summation of the offset signal and the current sensing signalis provided to an input 832 of the comparator 830. For example, thesummation is in the voltage domain. At the comparator 830, the summationis compared with a predetermined threshold signal generated by thethreshold generator 860. Based on the comparison, the comparator 830sends a signal to the PWM generator 840. For example, the PWM generator840 includes a PWM comparator 842, a logic controller 844, and a gatedriver 846. The logic controller receives the signal sent from thecomparator 830. In another example, the PWM comparator 842 receives theclock signal and the ramping signal generated by the oscillator. The PWMgenerator 840 receives the signal from the comparator 830 and controlsthe switch 870 through the gate driver 846. The effects of “delay tooutput” can be compensated by adjusting the resistor R. For example, thesystem 800 allows a maximum current that is constant over a wide rangeof the input voltage V_(in). In another example, the resistor 810 isadjusted for given L_(p) and R_(s) in a switching mode converteraccording to Equation 20. In yet another example, the brownoutprotection system 854 receives the voltage βV_(in), and is used toprotect a switch-mode converter if an input voltage falls below apredetermined value.

FIG. 9 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 900 includesresistors 910, 911, 912, and 914, a comparator 930, apulse-width-modulation (PWM) generator 940, a sensing system 950, atransconductor 952, a threshold generator 960, and a switch 970.Although the above has been shown using a selected group of componentsfor the system 900, there can be many alternatives, modifications, andvariations. For example, some of the components may be expanded and/orcombined. Other components may be inserted to those noted above. Forexample, the system 900 includes an oscillator, which sends a clocksignal and a ramping signal to the PWM generator 940. In anotherexample, the system 900 includes a primary winding 982 with aninductance value of L_(p). In yet another example, the system 900includes a brownout protection system 954. In yet another embodiment,the system 900 includes a signal generator 962 for providing a voltagereference signal. Depending upon the embodiment, the arrangement ofcomponents may be interchanged with others replaced. For example, thesystem 900 is used to regulate a power converter. Further details ofthese components are found throughout the present specification and moreparticularly below.

For example, an input voltage V, at node 990 is received by a voltagedivider to generate a voltage βV_(in). For example, the voltage dividerincludes the resistors 911 and 912, which are external to the chip forPWM control. The voltage βV_(in) is received by the sensing system 950to generate a voltage signal. The voltage signal is sent to thetransconductor 952, which generates a current I_ _(vin) flowing throughthe resistor 910 of R and generating an offset signal. For example, thetransconductor 852 is a voltage-controlled current source. In anotherexample, the offset signal is an offset voltage. The offset signal isprovided to the threshold generator 960, which also receives a voltagereference signal from the signal generator 962. The threshold generator960 provides an effective threshold signal V_(th) _(_) _(oc) _(_) _(eff)to an input 934 of the comparator 930. Additionally, a current sensingsignal is received by an input 932 of the comparator 930. For example,the current sensing signal is generated by the resistor 914 of R_(s). Inanother example, the current sensing signal is in the voltage domain.

At the comparator 930, the current sensing signal is compared with theeffective threshold signal V_(th) _(_) _(oc) _(_) _(eff). Based on thecomparison, the comparator 930 sends a signal to the PWM generator 940.For example, the PWM generator 940 includes a PWM comparator 942, alogic controller 944, and a gate driver 946. The logic controllerreceives the signal sent from the comparator 930. In another example,the PWM comparator 942 receives the clock signal and the ramping signalgenerated by the oscillator. The PWM generator 940 receives the signalfrom the comparator 930 and controls the switch 970 through the gatedriver 946. The effects of “delay to output” can be compensated byadjusting the resistor R. For example, the system 900 allows a maximumcurrent that is constant over a wide range of the input voltage V_(in).In another example, the resistor R is adjusted for given L_(p) and R_(s)in a switching mode converter according to Equation 20. In yet anotherexample, the brownout protection system 954 receives the voltageβV_(in), and is used to protect a switch-mode converter if an inputvoltage falls below a predetermined value.

According to other embodiments of the present invention, an inputvoltage is sensed based on the maximum width of PWM signal. For example,the PWM signal is applied to the gate of a power switch in series to theprimary winding of a power converter. FIG. 10 is a simplified diagramshowing relationship between PWM signal maximum width and input voltageaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 5, themaximum current is constant with respect to input voltage, and themaximum width of PWM signal varies with the input voltage. For example,the input voltage is the input line voltage for power converter. Inanother example, the maximum current I_(PEAK1) equals the maximumcurrent I_(PEAK2). The maximum current I_(PEAK1) corresponds to a higherinput voltage and a PWM signal 510, and the maximum current I_(PEAK2)corresponds to a lower input voltage and a PWM signal 520. As shown inFIG. 5, the maximum width for the PWM signal 510 is narrower for higherinput voltage, and the maximum width for PWM signal 520 is wider forlower input voltage. The input voltage is represented by the maximumwidth of PWM signal if the maximum current is constant with respect toinput voltage. Accordingly, the maximum width of PWM signal can be usedto determine the threshold offset to compensate for the effects of“delay to output” as shown in Equation 6.

In one embodiment, the compensation can be realized by generating acurrent threshold, I_(th) _(_) _(oc), which is a function of the maximumwidth of PWM signal as shown in FIG. 10. For example, the currentthreshold is equal to I_(th) _(_) _(oc) _(_) ₁ for the PWM signal 510and I_(th) _(_) _(oc) _(_) ₂ for the PWM signal 520. In another example,the slope of I_(th) _(_) _(oc) with respect to the maximum width isproperly chosen to compensate for the effects of “delay to output”according to Equation 6. The selected slope takes into accountinformation about power converter components that are external to thechip for PWM control. For example, the external components include theprimary winding, a current sensing resistor and a power MOSFET. Thisembodiment of the present invention includes certain examples of FIG.11.

FIG. 11 is a simplified control system with constant maximum currentaccording to yet another embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. A system 1100 includesresistors 1112, 1114, and 1116, a current supplier 1120, a startupsystem 1152, a comparator 1130, a pulse-width-modulation (PWM) generator1140, a threshold generator 1160, and a switch 1170. Although the abovehas been shown using a selected group of components for the system 1100,there can be many alternatives, modifications, and variations. Forexample, some of the components may be expanded and/or combined. Othercomponents may be inserted to those noted above. For example, the system1100 includes an oscillator 1180, which sends a clock signal and aramping signal to the PWM generator 1140. In another example, the system1100 includes a primary winding 1182 with an inductance value of L_(p).Depending upon the embodiment, the arrangement of components may beinterchanged with others replaced. For example, the system 1100 is usedto regulate a power converter. In another example, the resistor 1116 ofR_(comp) is removed. Further details of these components are foundthroughout the present specification and more particularly below.

The current supplier 1120 is connected to the resistor 1116 of R_(comp),and is used to generate a current I_ _(comp) . For example, the currentsupplier 1120 is a current sink. In another example, the currentsupplier 1120 includes a transconductor. In yet another example, thecurrent I_ _(comp) is a ramping current that is synchronized with a PWMsignal. The current I_ _(comp) flows through the resistor 1116 ofR_(comp) and the resistor 1110 of R and generates an offset signal. Forexample, the offset signal is in the voltage domain. The offset signalis subtracted from a current sensing signal to generate a compositesignal. For example, the current sensing signal is generated by theresistor 1114 of R_(s). The composite signal is provided to an input1132 of the comparator 1130. For example, the composite signal is in thevoltage domain and represented by V_(com). At the comparator 1130, thecomposite signal is compared with a predetermined threshold signalgenerated by the threshold generator 1160. For example, thepredetermined threshold signal is the threshold voltage V_(th) _(_)_(oc). Based on the comparison, the comparator 1130 sends a signal tothe PWM generator 1140. For example, the PWM generator 1140 includes aPWM comparator 1142, a logic controller 1144, and a gate driver 1146.The logic controller receives the signal sent from the comparator 1130.In another example, the PWM comparator 1142 receives the clock signaland the ramping signal generated by the oscillator 1180. The PWMgenerator 1140 receives the signal from the comparator 1130 and controlsthe switch 1170 through the gate driver 1146. For example, the gatedriver 1146 generates the PWM signal. The startup system 1152 isconnected to the input voltage V_(in) through the resistor 1112, and isused to control powering up of a chip for PWM control.

In one embodiment, the voltage V_(con), at the input 1132 of thecomparator 1130 isV _(com) =I _(sense) ×R _(s) −I_ _(comp) ×(R _(comp) +R)  (Equation 21)

The comparator 1130 sends a signal to the PWM generator 1140 for turningoff the switch 1170 if V_(com) reaches the threshold voltage V_(th) _(_)_(oc) as follows:I _(sense) ×R _(s) −I_ _(comp) ×(R _(comp) +R)=V _(th) _(_)_(oc)  (Equation 22)and I _(sense) ×R _(s) =V _(th) _(_) _(oc) +I_ _(comp) ×(R _(comp)+R)  (Equation 23)

The effective threshold signal l_(th) _(_) _(oc) is

$\begin{matrix}{I_{th\_ oc} = \frac{I_{sense}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 24} \right) \\{{Therefore},\text{}{I_{th\_ oc} = {\frac{V_{th\_ oc}}{R_{s}} + \frac{I_{\_ comp} \times \left( {R_{comp} + R} \right)}{R_{s}}}}} & \left( {{Equation}\mspace{14mu} 25} \right)\end{matrix}$

where I_ _(comp) is a ramping signal whose amplitude is synchronizedwith the PWM signal. Different maximum widths of the PWM signal resultsin different magnitudes for the second term in Equation 25. For example,the larger maximum width corresponds to lower input voltage, and thesmaller maximum width corresponds to higher input voltage. Accordingly,the higher input voltage results in smaller I_(th) _(_) _(oc), and thelower input voltage results in larger I_(th) _(_) _(oc). In anotherexample, the ramping signal I_ _(comp) is described as follows:I_ _(comp) (t)=δ×(t−nT)  (Equation 26)and 0≦t≦T _(on)  (Equation 27)

where δ is a constant, and T_(on) is the maximum width of the PWMsignal. For example, T_(on) corresponds to a period during which the PWMsignal turns on the switch 1170. In another example, a period of the PWMsignal T_(on) and T_(off) during which the PWM signal turns off theswitch 1170. In yet another example, T_(on) depends on the input voltageand the maximum current, and is represented by T_ _(vin) correspondingto the input voltage V_(in).

Accordingly, at t=T _(on) (Equation 28)

$\begin{matrix}{I_{th\_ oc} = {{\frac{V_{th\_ oc}}{R_{s}} + \frac{I_{\_ comp} \times \left( {R_{comp} + R} \right)}{R_{s}}} = {\frac{V_{th\_ oc}}{R_{s}} + \frac{\delta \times T_{\_ vin} \times \left( {R_{comp} + R} \right)}{R_{s}}}}} & \left( {{Equation}\mspace{14mu} 29} \right)\end{matrix}$

For example, T_ _(vin) can be expressed as follows:

$\begin{matrix}{{T_{\_ vin} = \frac{V_{th\_ oc} \times L_{p}}{V_{in} \times R_{s}}}{Hence}} & \left( {{Equation}\mspace{14mu} 30} \right) \\{I_{th\_ oc} = {\frac{V_{th\_ oc}}{R_{s}} + \frac{\delta \times \left( {R_{comp} + R} \right) \times V_{th\_ oc} \times L_{p}}{V_{in} \times R_{s} \times R_{s}}}} & \left( {{Equation}\mspace{14mu} 31} \right)\end{matrix}$

As shown in Equation 31, the second term on the right side is inverselyproportional to V_(in). The effective threshold is lower for higherinput voltage and higher for lower input voltage. By adjusting theresistor 1110 of R, the effects of “delay to output” can be compensated.For example, the system 1100 allows a maximum current that is constantover a wide range of the input voltage V_(in). In another example, theresistor 1110 is adjusted for given L_(p) and R_(s) in a switching modeconverter according to Equation 31.

As discussed above and further emphasized here, Equations 1-31 aremerely examples, which should not unduly limit the scope of the claims.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. For example, Equations 7-31 are used todescribe certain examples for FIGS. 5-9 and 11, but FIGS. 5-9 and 11 canoperate according to methods that may be different from those ofEquations 7-31.

According to another embodiment of the present invention, a programmableline-voltage-compensated current-limiting control system and a methodthereof are provided. The line input voltage is sensed and received byan input-controlled current source. The current source generates acurrent that flows through an external resistor connecting between acurrent sensing terminal and a current sensing resistor R. The resultingoffset voltage is proportional to the line input voltage, and issuperimposed with a current sensing signal. The summation signal isprovided to an over-current comparator to generate a control signal. Thecontrol signal can be used to turn off a power switch in a switchingmode converter. For example, the system and the method are implementedaccording to the systems 500 and/or 600.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage is sensed andreceived by an input-controlled current source. The current source flowsthrough an external resistor connecting between the current source andthe ground. The resulting offset voltage is proportional to the lineinput voltage, and is superimposed with a reference signal to generatean effective threshold signal. The effective threshold signal iscompared with a current sensing signal by an over-current comparator togenerate a control signal. The control signal can be used to turn off apower switch in a switch mode converter. For example, the system and themethod are implemented according to the system 700.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage is sensed by aresistor and sensing transistors connecting between the line voltage anda chip power supply terminal. As a result, a current is generated thatis proportional to the line input voltage. The current is mirrored by acurrent mirror and/or a current amplifier to generate aninput-controlled current. The input-controlled current flows through anexternal resistor to generate an offset signal. For example, the systemand the method are implemented according to the systems 500, 600, and/or700.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage is divided bya voltage divider. The divide voltage is sensed and converted to aninput-controlled current by a transconductor. The input-controlledcurrent flows through an external resistor connecting between a currentsensing terminal and a current sensing resistor. The resulting offsetvoltage is proportional to the line input voltage, and is superimposedwith a current sensing signal. The summation signal is provided to anover-current comparator to generate a control signal. The control signalcan be used to turn off a power switch in a switch mode converter. Forexample, the system and the method are implemented according to thesystem 800.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage is divided bya voltage divider. The divide voltage is sensed and converted to aninput-controlled current by a transconductor. The input-controlledcurrent source flows through an external resistor connecting between thetransconductor and the ground. The resulting offset voltage isproportional to the line input voltage, and is superimposed with areference signal to generate an effective threshold signal. Theeffective threshold signal is compared with a current sensing signal byan over-current comparator to generate a control signal. This controlsignal can be used to turn off a power switch in a switch modeconverter. For example, the system and the method are implementedaccording to the system 900.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage is divided bya voltage divider. The divided voltage is sensed and converted by atransconductor. As a result, a current is generated that is proportionalto the line input voltage. The current is mirrored by a current mirrorand/or a current amplifier to generate an input-controlled current. Theinput-controlled current flows through an external resistor to generatean offset signal. For example, the system and the method are implementedaccording to the systems 800 and/or 900.

According to yet another embodiment of the present invention, aprogrammable line-voltage-compensated current-limiting control systemand a method thereof are provided. The line input voltage information isrepresented by the PWM width for constant current limiting. APWM-synchronized current-ramping signal controls a current sinkconnected to a current sensing terminal of a chip. As a result, acurrent that flows through an external resistor into the chip and flowsthrough an internal resistor in the chip is sunk by the current sink.Additionally, the current has a ramping magnitude that is synchronizedwith a PWM signal. The resulting offset voltage is subtracted from acurrent sensing signal to generate a composite signal. The compositesignal is provided to an over-current comparator to generate a controlsignal. The control signal can be used to turn off a power switch in aswitch mode converter. For example, the system and the method areimplemented according to the system 1100.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator configured to receive a thresholdsignal and a first signal and to generate a comparison signal. The firstsignal is a sum of a second signal and a third signal, and the thirdsignal is associated with an input current for a power converter.Additionally, the system includes a pulse-width-modulation generatorconfigured to receive the comparison signal and generate a modulationsignal in response to the comparison signal, and a switch configured toreceive the modulation signal and control the input current for thepower converter. An amplitude for the first signal becomes larger if anamplitude for the input voltage becomes larger. The second signal isgenerated by receiving an input voltage for the power converter,converting the received input voltage to a fourth signal, and convertingthe fourth signal to the second signal. For example, the system isimplemented according to the systems 500, 600, and/or 800.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator configured to receive a firstsignal and a second signal and to generate a comparison signal. Thefirst signal is associated with an input current for a power converter.Additionally, the system includes a threshold generator configured toreceive at least a third signal and generate the second signal inresponse to at least the third signal. The third signal is associatedwith an input voltage for the power converter. Moreover, the systemincludes a pulse-width-modulation generator configured to receive thecomparison signal and generate a modulation signal in response to thecomparison signal, and a switch configured to receive the modulationsignal and control the input current for the power converter. Anamplitude for the second signal becomes smaller if an amplitude for theinput voltage becomes larger, and the third signal is generated byreceiving an input voltage for the power converter and converting thereceived input voltage to the third signal. For example, the system isimplemented according to the systems 700 and/or 900.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator configured to receive a thresholdsignal and a first signal and to generate a comparison signal. The firstsignal is equal to a second signal subtracted by a third signal, and thesecond signal is associated with an input current for a power converter.Additionally, the system includes a pulse-width-modulation generatorconfigured to receive the comparison signal and generate a modulationsignal in response to the comparison signal, and a switch configured toreceive the modulation signal and control the input current for thepower converter. Moreover, the system includes an oscillator coupled tothe pulse-width-modulation generator and configured to generate at leasta first control signal, a transconductor configured to receive the firstcontrol signal and generate a second control signal, and a currentsupplier configured to receive the second control signal and generate afirst current in response to the second control signal, the firstcurrent being associated with the third signal. An amplitude for thefirst current becomes smaller if an amplitude for the input voltagebecomes larger. For example, the system is implemented according to thesystem 1100.

According to yet another embodiment, a method for protecting a powerconverter includes receiving an input voltage for a power converter,converting the received input voltage to a first signal, converting thefirst signal to a second signal, and generating a third signal based onat least information associated with the second signal. Additionally,the method includes receiving the third signal and a threshold signal.The third signal is a sum of the second signal and a fourth signal, andthe fourth signal is associated with an input current for the powerconverter. Moreover, the method includes generating a comparison signalbased on at least information associated with the third signal and thethreshold signal, and processing information associated with thecomparison signal. Also, the method includes generating a modulationsignal based on at least information associated with the comparisonsignal, and controlling the input current for the power converter inresponse to the modulation signal. An amplitude for the third signalbecomes larger if an amplitude for the input voltage becomes larger. Forexample, the method is implemented by the systems 500, 600, and/or 800.

According to yet another embodiment, a method for protecting a powerconverter includes receiving an input voltage for a power converter,converting the received input voltage to a first signal, processinginformation associated with the first signal, generating a second signalbased on at least information associated with the first signal, andreceiving the second signal and a third signal. The third signal isassociated with an input current for a power converter. Additionally,the method includes generating a comparison signal based on at leastinformation associated with the second signal and the third signal,processing information associated with the comparison signal, generatinga modulation signal based on at least information associated with thecomparison signal, and controlling the input current for the powerconverter in response to the modulation signal. An amplitude for thesecond signal becomes smaller if an amplitude for the input voltagebecomes larger. For example, the method is implemented by the systems700 and/or 900.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first signal based on at leastinformation associated with an input current for a power converter,generating a second signal, the second signal being proportional to aramping current, and processing information associated with the firstsignal and the second signal. Additionally, the method includesgenerating a third signal equal to the first signal subtracted by thesecond signal, receiving the third signal and a threshold signal,generating a comparison signal based on at least information associatedwith the third signal and the threshold signal, processing informationassociated with the comparison signal, generating a modulation signalbased on at least information associated with the comparison signal, andcontrolling the input current for the power converter in response to themodulation signal. An amplitude for the ramping current corresponding toa predetermined value for the input current becomes smaller if anamplitude for the input voltage becomes larger. For example, the methodis implemented by the system 1100.

The present invention has various applications. In some embodiments, thesystems of FIGS. 5-9 and/or 11 can be used to regulate switch-modeconverters. For example, the switch-mode converters include offlineflyback converters and/or forward converters. In other embodiments, thesystems of FIGS. 5-9 and/or 11 allow a maximum power that is constantover a wide range of input voltage.

The present invention has various advantages. Some embodiments canprovide an excellent compensation for the “delay to output” by easilyadjusting an external resistor. For example, the adjustment of theexternal resistor takes into account converter components external to achip for PWM control. Certain embodiments allow a maximum current and amaximum power that are constant over a wide range of input voltage. Someembodiments consume a low standby power by sharing a resistor for asensing system with a startup system and/or a brownout protectionsystem. For example, the resistor is shared by a sensing system and astartup system. Certain embodiments provide an excellent compensationfor the “delay to output” without sensing an input voltage. For example,the pin counts for a chip for PWM control is limited. In anotherexample, the maximum width of a PWM signal is used to represent theinput voltage. Different input voltages result in different maximumwidths for the PWM signal, and the different maximum widths result indifferent effective threshold signals. Some embodiments provide anover-current protection that can effectively protect a power converterfrom excessive power, thermal run away, excessive current and/or voltagestress.

To achieve high efficiency, a power converter usually works in CCM modeat low input line voltage and works in DCM mode at high input linevoltage. FIG. 12 shows simplified current profiles for primary windingin CCM mode and DCM mode. The current profiles describe currentmagnitudes as functions of time. As shown in FIG. 12(a), the current forprimary winding increases from I_L to a current limit I_p1 within apulse width at each cycle in DCM mode. For example, I_L is equal tozero. The energy delivered to the load at each cycle is

$\begin{matrix}{ɛ = {\frac{1}{2} \times L_{p} \times \left( {{I\_ p}\; 1} \right)^{2}}} & \left( {{Equation}\mspace{14mu} 32} \right)\end{matrix}$

In contrast, as shown in FIG. 12(b), the current for primary windingincreases from I_i2 to a current limit I_p2 within a pulse width at eachcycle in CCM mode. For example, I_i2 is larger than zero. The energydelivered to the load at each cycle is

$\begin{matrix}{ɛ = {\frac{1}{2} \times L_{p} \times \left\lbrack {\left( {{I\_ p}\; 2} \right)^{2} - \left( {{I\_ i}\; 2} \right)^{2}} \right\rbrack}} & \left( {{Equation}\mspace{14mu} 33} \right)\end{matrix}$

where the ratio of

$\frac{{I\_ i}\; 2}{{I\_ p}\; 2}$can vary with input line voltage. For example, the ratio increases withdecreasing input line voltage. As described in Equations 32 and 33, ifthe two current limits I_p1 and I_p2 are equal, the amount of energydelivered to the load in DCM mode is higher than the amount of energydelivered to the load in CCM mode at each cycle.

FIG. 13 shows a simplified diagram for maximum energy delivered to loadat each cycle as a function of input line voltage. As a function ofinput line voltage, the current limit, which equals either I_p1 or I_p2,is adjusted to compensate for “delay to output” as shown in FIG. 4, butdifferences between Equations 32 and 33 have not been taken intoaccount. Also, FIG. 4 does not appear to have taken into account thevarying ratio of

$\frac{{I\_ i}\; 2}{{I\_ p}\; 2}.$Hence the maximum energy is not constant over the entire range of lineinput voltage. For example, as shown by a curve 1300, the maximum energydecreases significantly with decreasing input line voltage in CCM mode,even though the maximum energy appears substantially constant in the DCMmode.

To achieve constant maximum energy over a wide range of input linevoltage, the following should be satisfied based on Equations 32 and 33:

$\begin{matrix}{{\frac{1}{2} \times L_{p} \times \left( {{I\_ p}\; 1} \right)^{2}} = {\frac{1}{2} \times L_{p} \times \left\lbrack {\left( {I - {p\; 2}} \right)^{2} - \left( {{I\_ i}\; 2} \right)^{2}} \right\rbrack}} & \left( {{Equation}\mspace{14mu} 34} \right)\end{matrix}$

In practice, the power converter works in different modes and withdifferent PWM pulse widths for a given load at different input linevoltages. FIG. 14 is a simplified diagram showing relationship betweencurrent limit and PWM pulse width according to an embodiment of thepresent invention. The PWM pulse width is represented by a time period.FIG. 14 is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications.

The current limit is represented by I_p1 in DCM mode and by I_p2 in CCMmode. For example, I_p1 has a linear relationship with pulse widthaccording to a slope S_p1, and I_p2 has a linear relationship with pulsewidth according to a slope S_p2. In one embodiment, the slope S_p1 issmaller than the slope S_p2. In another embodiment, I_p1 and I_p2 areequal at the pulse width Ton_0. For example, the pulse width Ton_0represents a transition between DCM mode and CCM mode. In yet anotherembodiment, the pulse width reflects different values for input linevoltage; hence the pulse width can be used to make the current limitdepend on the input line voltage.

For example, at a high input line voltage, such as 264 volts in AC, thepower converter works with narrow pulse width Ton_1 in the DCM mode toprovide full power demanded by the load. As shown in FIG. 14, theprimary winding current is limited to I_p1. Within each pulse width, thecurrent for primary winding increases from I_L to the current limitI_p1. In another example, at a low input line voltage, such as 90 voltsin AC, the power converter works with wide pulse width Ton_2 in the CCMmode to provide full power demanded by the load. As shown in FIG. 14,the primary winding current is limited to I_p2. Within each pulse width,the current for primary winding increases from I_i2 to the current limitI_p2.

FIGS. 15(A) and (B) are simplified diagrams showing relationship betweenover-current threshold voltage with pulse width and input line voltagerespectively according to an embodiment of the present invention. Forexample, the over-current threshold voltage V_(th) _(_) _(OC) is used tocontrol the peak current for the primary winding as shown in FIG. 1.FIGS. 15(A) and (B) are merely examples, which should not unduly limitthe scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

As shown in FIG. 15(A), the over-current threshold voltage V_(th) _(_)_(OC) has different relationships with the PWM pulse width in differentregions. In region I, the threshold voltage V_(th) _(_) _(OC) has alinear relationship with pulse width according to a slope S_1 w; inregion II, the threshold voltage V_(th) _(_) _(OC) has a linearrelationship with pulse width according to a slope S_2 w; and in regionIII, the threshold voltage V_(th) _(_) _(OC) has a linear relationshipwith pulse width according to a slope S_3 w. For example, the slope S_2w is larger than the slope S_1 w in magnitude, and the slope S_1 w islarger than the slope S_3 w in magnitude. In one embodiment, the slopeS_3 w is equal to zero. In another embodiment, the decreasing of the PWMpulse width corresponds to the increasing of the input line voltage forfull power demanded by the load at a given switching frequency.

As shown in FIG. 15(B), the over-current threshold voltage V_(th) _(_)_(OC) has different relationships with input line voltage in differentregions. In region I, the threshold voltage V_(th) _(_) _(OC) has alinear relationship with input line voltage according to a slope S_1 v;in region II, the threshold voltage V_(th) _(_) _(OC) has a linearrelationship with input line voltage according to a slope S_2 v; and inregion III, the threshold voltage V_(th) _(_) _(OC) has a linearrelationship with input line voltage according to a slope S_3 v. Forexample, the slope S_1 v corresponds to the slope S_3 w, the slope S_2 vcorresponds to the slope S_2 w, and the slope S_3 v corresponds to theslope S_1 w. In another example, the slope S_2 w is larger than theslope S_3 w in magnitude, and the slope S_3 w is larger than the slopeS_1 w in magnitude. In one embodiment, the slope S_1 w is equal to zero.In another embodiment, the slope S_3 w is equal to

$\frac{T_{delay}}{L_{p}}$as shown in FIG. 4.

According to one embodiment, in region I, the power converter works inthe DCM mode for the power demand up to the full load at input linevoltage in a relatively high range. The over-current threshold voltageV_(th) _(_) _(OC) increases with the decreasing input line voltage orthe increasing PWM pulse width for given switching frequency in order tocompensate for propagation delay, T_(delay). According to anotherembodiment, in region II, the power converter works in the CCM mode forthe power demand up to the full load at input line voltage in an intermediate range. The over-current threshold voltage V_(th) _(_) _(OC)increases with the decreasing input line voltage or the increasing PWMpulse width for given switching frequency in order to compensate forboth CCM mode operation and propagation delay, T_(delay). According toyet another embodiment, in region III, the power converter works in theCCM mode, but the over-current threshold voltage V_(th) _(_) _(oc) islimited to Vth_OC_2 for input line voltage in a relatively low range.For example, the limitation of Vth_OC_2 can protect the primary windingcurrent from rising too high or prevent saturation of the powerconverter inductor.

FIG. 16 shows a simplified diagram for maximum energy delivered to loadat each cycle as a function of input line voltage according to certainembodiments of the present invention. As shown by a curve 1600, themaximum energy, for example, remains substantially flat with decreasinginput line voltage in CCM mode. In comparison, the curve 1300 shows themaximum energy decreases significantly with decreasing input linevoltage in CCM mode. In one embodiment, the curve 1600 describes theperformance of the system as shown in FIG. 17. In another example, thecurve 1600 describes the performance of the system as shown in FIG. 19.

FIG. 17 is a simplified control system for over-current and over-powerprotection according to yet another embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. A system 1700 includesa signal generator 1710, a pulse-width-modulation (PWM) generator 1720,comparators 1730, 1740, and 1750, and a threshold generator 1760.Although the above has been shown using a selected group of componentsfor the system 1700, there can be many alternatives, modifications, andvariations. For example, some of the components may be expanded and/orcombined. Other components may be inserted to those noted above.Depending upon the embodiment, the arrangement of components may beinterchanged with others replaced. For example, the system 1700 is usedto regulate a power converter. Further details of these components arefound throughout the present specification and more particularly below.

The signal generator 1710 generates a clock signal 1712 and a rampingsignal 1714. For example, the clock signal 1712 is periodic, each periodincluding an off period and an on period. The clock signal 1712 and theramping signal 1714 are sent to the PWM generator 1720. For example, thesignal generator 1710 is an oscillator. In another example, the rampingsignal 1714 represents voltage level as a function of time. The PWMgenerator 1720 generates a PWM signal 1722. In one embodiment, the PWMsignal 1722 is used to control a switch for current flowing through aprimary winding of a voltage converter

As shown in FIG. 17, the ramping signal 1714 is also received by thecomparators 1730 and 1740. For example, the comparator 1730 compares theramping signal 1714 and a reference voltage V_(th) _(_) ₁, and generatesa comparison signal 1732. In another example, the comparator 1740compares the ramping signal 1714 and a reference voltage V_(th) _(_) ₂,and generates a comparison signal 1742.

Both the comparison signals 1732 and 1742 are received by the thresholdgenerator 1760. The threshold generator 1760 also receives the rampingsignal 1714. Based on at least information associated with thecomparison signals 1732 and 1742, and the ramping signal 1714, thethreshold generator 1760 outputs a voltage signal 1762. For example, thevoltage signal 1762 represents the over-current threshold voltage V_(th)_(_) _(OC) as a function of time. In another example, the voltage signal1762 is a periodic signal with a period T. According to one embodiment,within each period, the voltage signal 1762 covers three regions I, II,and III, as shown in FIG. 15(A). According to another embodiment, withineach period, the voltage signal 1762 can be used to determine the valueof the threshold voltage V_(th) _(_) _(OC) for a given pulse width asshown in FIGS. 14 and 15. For example, different pulse widths correspondto different input line voltages at a give switching frequency for agiven load.

The voltage signal 1762 is sent by the threshold generator 1760 to thecomparator 1750, which also receives a current sensing signal 1752. Inone embodiment, the current sensing signal 1752 is a ramping signal thatrepresents a current-sensing voltage V_(CS). For example, thecurrent-sensing voltage V_(CS) increases linearly with time according toa slope

$\frac{V_{in}}{L_{p}}{R_{s}.}$R_(s) is the resistance for a resistor 1760, and L_(p) is the inductanceof the primary winding for the power converter. In another example, thecurrent-sensing voltage V_(CS) is determined byV _(CS) =I _(sense) ×R _(s)  (Equation 35)

where I_(sense) represents the current flowing through the primarywinding.

The comparator 1750 compares the voltage signal 1762 and the currentsensing signal 1752, and generates an output signal 1754. For example,at the beginning of each period T, the switch for the primary winding isclosed, e.g., turned on. Then if V_(CS) becomes equal to or larger thanV_(th) _(_) _(OC), the output signal 1754 becomes logic low. As shown inFIG. 17, the output signal 1754 is received by the PWM generator 1720.If the output signal 1754 becomes logic low, the PWM signal turns offthe switch for the current flowing through the primary winding accordingto an embodiment. Therefore, different peak currents for the primarywinding can be achieved as a function of input line voltage and as afunction of PWM pulse width.

Specifically, according to certain embodiments, the peak current for theprimary winding is determined by

$\begin{matrix}{I_{peak} = \frac{V_{th\_ OC}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 36} \right)\end{matrix}$

For example, the threshold voltage V_(th) _(_) _(OC) is periodic withthe period T and covers three regions I, II, and III within each period.For example, each period starts at t₀ and ends at t₀+T. Period I extendsfrom t₀ to t₁, period II extends from t₁ to t₂, and period III extendsfrom t₂ to t₀+T. In one embodiment, the over-current threshold voltageV_(th) _(_) _(OC) is determined byRegion I: V _(th) _(_) _(OC) =V _(th) _(_) _(OC) _(_) ₀+β₁×(t−t ₀) t ₀≦t≦ ₁  (Equation 37A)Region II: V _(th) _(_) _(OC) =V _(th) _(_) _(OC) _(_) ₀+β₁×(t ₁ −t₀)+β₂×(t−t ₁) t ₁ ≦t≦ ₂  (Equation 37B)Region III: V _(th) _(_) _(OC) =V _(th) _(_) _(OC) _(_) ₀+β₁×(t ₁ −t₀)+β₂×(t ₂ −t ₁) t ₂ ≦t≦ ₀ +T  (Equation 37C)

where β₁ and β₂ are the slopes. For example, as shown in FIG. 15(A),β₁ =S_1w  (Equation 38A)β₂ =S_2w  (Equation 38B)

As discussed above and further emphasized here, FIG. 17 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the comparator 1750 includes two or morecomparators. In another example, the comparator 1750 also includes oneor more logic component, such as an AND gate.

FIG. 18 is a simplified diagram showing the threshold generator 1760 andthe comparator 1750 in the control system 1700 for over-current andover-power protection according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Thethreshold generator 1760 includes current sources 1910 and 1920,transistors 1930, 1940, and 1950, a capacitor 1960, comparators 1970,1972, and 1974, an OR gate 1980, and an AND gate 1990. Although theabove has been shown using a selected group of components for thethreshold generator 1760, there can be many alternatives, modifications,and variations. For example, some of the components may be expandedand/or combined. Other components may be inserted to those noted above.Depending upon the embodiment, the arrangement of components may beinterchanged with others replaced. Further details of these componentsare found throughout the present specification and more particularlybelow.

During the off period of the clock signal 1712, the transistor 1950 isturned on, and the transistors 1930 and 1940 are turned off. Forexample, the transistor 1950 is the NMOS transistor MN1, and thetransistors 1930 and 1940 are PMOS transistors MP1 and MP2 respectively.As a result, the voltage across the capacitor 1960 is V_(th) _(_) _(oc)_(_) ₀.

During the on period of the clock signal 1712, the transistor 1950 isturned off. Between t₀ and t₁, the signal 1732 is at logic high, and thesignal 1742 is at logic low. As a result, the transistor 1930 is turnedon but the transistor 1940 remains off. As shown in FIG. 18, thecapacitor 1960 is charged by a current 1912 generated by the currentsource 1910. In one embodiment, the current 1912 is equal to I_(c), andthe capacitance of the capacitor 1960 is equal to C₀. The voltage acrossthe capacitor 1960 ramps up at the slop of

$\beta_{1} = \frac{I_{c}}{C_{o}}$as shown in Equation 37A. The voltage across the capacitor 1960 is thethreshold voltage for the comparator 1972. In one embodiment, thecomparator 1972 compares the threshold voltage and the current sensingsignal 1752, and generates a comparison signal. If V_(CS) becomes equalto or larger than the threshold voltage, the output signal 1754 becomeslogic low. If the output signal 1754 becomes logic low, the PWM signalturns off the switch for the current flowing through the primary windingaccording to an embodiment.

Between t₁ and t₂ during the on period of the clock signal 1712, bothsignals 1732 and 1742 are at logic high. Thus, the transistors 1930 and1940 are turned on. The capacitor 1960 is charged with the current 1912and another current 1922 that is generated by the current source 1920.In one embodiment, the current 1912 is equal to I_(c), and the current1922 is equal to k×I_(c). The voltage across the capacitor 1960 ramps upat the slop of

$\beta_{2} = \frac{\left( {K + 1} \right) \times I_{c}}{C_{o}}$as shown in Equation 37B. The voltage across the capacitor 1960 is thethreshold voltage for the comparator 1972. In one embodiment, thecomparator 1972 compares the threshold voltage and the current sensingsignal 1752, and generates a comparison signal. If V_(CS) becomes equalto or larger than the threshold voltage, the output signal 1754 becomeslogic low. If the output signal 1754 becomes logic low, the PWM signalturns off the switch for the current flowing through the primary windingaccording to an embodiment. After t₂, whenever V_(CS) becomes equal toor larger than V_(th) _(_) _(oc) _(_) ₂, the output of the comparator1974 becomes logic low. As a result, the signal 1754 is at logic lowregardless of the output of the comparator 1972. Hence, for certainembodiments, the over-current threshold voltage V_(th) _(_) _(OC)saturates at V_(th) _(_) _(oc) _(_) ₂ as shown in FIG. 15(A).

FIG. 19 is a simplified control system for over-current and over-powerprotection according to yet another embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. A system 1800 includesa signal generator 1810, a pulse-width-modulation (PWM) generator 1820,comparators 1830, 1840, and 1850, a current generator 1860, and aresistor 1870. Although the above has been shown using a selected groupof components for the system 1800, there can be many alternatives,modifications, and variations. For example, some of the components maybe expanded and/or combined. Other components may be inserted to thosenoted above. Depending upon the embodiment, the arrangement ofcomponents may be interchanged with others replaced. For example, thesystem 1800 is used to regulate a power converter. Further details ofthese components are found throughout the present specification and moreparticularly below.

The signal generator 1810 generates a clock signal 1812 and a rampingsignal 1814. The clock signal 1812 and the ramping signal 1814 are sentto the PWM generator 1820. For example, the signal generator 1810 is anoscillator. In another example, the ramping signal 1814 representsvoltage level as a function of time. The PWM generator 1820 generates aPWM signal 1822. In one embodiment, the PWM signal 1822 is used tocontrol a switch for current flowing through a primary winding of avoltage converter.

As shown in FIG. 19, the ramping signal 1814 is also received by thecomparators 1830 and 1840. For example, the comparator 1830 compares theramping signal 1814 and a reference voltage V_(th) _(_) ₁, and generatesa comparison signal 1832. In another example, the comparator 1840compares the ramping signal 1814 and a reference voltage V_(th) _(_) ₂,and generates a comparison signal 1842.

Both the comparison signals 1832 and 1842 are received by the currentgenerator 1860. The current generator 1860 also receives the rampingsignal 1814. Based on at least information associated with thecomparison signals 1832 and 1842, and the ramping signal 1814, thecurrent generator 1860 generates a current 1862. For example, thecurrent 1862 flows into the current generator 1860. In another example,the current generator 1860 is a current sink. In yet another example,the current 1862 has a magnitude I_(comp) that varies as a function oftime. In one embodiment, the magnitude I_(comp) is periodic with aperiod T.

As shown in FIG. 19, the current generator 1860 is coupled to apredetermined voltage and the resistor 1870. In one embodiment, thepredetermined voltage is the ground voltage. In another embodiment, theresistor 1870 has a resistance value R_(comp). The current 1862 flowingthrough the resistor 1870 generates a compensation voltage V_(comp). Thecompensation voltage V_(comp) is superimposed with a current-sensingvoltage V_(CS), thus generating a total voltage V_(total). For example,the current-sensing voltage V_(CS) increases linearly with timeaccording to a slope

$\frac{V_{in}}{L_{p}}{R_{s}.}$R_(S) is the resistance for a resistor 1870, and L_(p) is the inductanceof the primary winding for the power converter. In another example, thecurrent-sensing voltage V_(CS) is determined by R_(S) and I_(sense)based on Equation 35, where I_(sense) represents the current flowingthrough the primary winding.

According to an embodiment, the total voltage V_(total) is equal to:V _(total) =V _(cs) −V _(comp) =V _(cs) −I _(comp) ×R _(comp) =I_(sense) ×R _(s) −I _(comp) ×R _(comp)  (Equation 39)

The total voltage V_(total) is received by the comparator 1850, whichalso receives a threshold voltage V_(th) _(_) _(OC) _(_) ₀. Thecomparator 1850 compares the total voltage V_(total) and the thresholdvoltage V_(th) _(_) _(OC) _(_) ₀, and generates an output signal 1854.For example, at the beginning of each period T, the switch for theprimary winding is closed, e.g., turned on. Then if V_(total) becomesequal to or larger than V_(th) _(_) _(OC) _(_) ₀, the output signal 1854becomes logic low. As shown in FIG. 19, the output signal 1854 isreceived by the PWM generator 1820. If the output signal 1854 becomeslogic low, the PWM signal would turn off the switch for the currentflowing through the primary winding according to an embodiment.Therefore, different peak current for the primary winding can beachieved as a function of input line voltage and as a function of PWMpulse width.

$\begin{matrix}{I_{peak} = \frac{V_{{th\_ OC}\_ 0} + {I_{comp} \times R_{comp}}}{R_{s}}} & \left( {{Equation}\mspace{14mu} 40} \right)\end{matrix}$

Hence the effective threshold voltage V_(th) _(_) _(OC) _(_) _(eff) isV _(th) _(_) _(OC) _(_) _(eff) =V _(th) _(_) _(OC) _(_) ₀ +I _(comp) ×R_(comp)  (Equation 41)

According to one embodiment, within each period, the effective thresholdvoltage V_(th) _(_) _(OC) _(_) _(eff) covers three regions I, II, andIII, as shown in FIG. 15(A). According to another embodiment, withineach period, Equation 41 can be used to determine the value of theeffective threshold voltage V_(th) _(_) _(OC) _(_) _(eff) for a givenpulse width as shown in FIGS. 14 and 15. For example, different pulsewidths correspond to different input line voltages at a give switchingfrequency for a given load.

As a result of Equation 41, I_(comp) can be, for example, determined by

$\begin{matrix}{I_{comp} = \frac{V_{{th\_ OC}{\_ eff}} - V_{{th\_ OC}\_ 0}}{R_{comp}}} & \left( {{Equation}\mspace{14mu} 42} \right)\end{matrix}$

According to certain embodiments, the current magnitude I_(comp) isperiodic with the period T and covers three regions I, II, and IIIwithin each period. For example, each period starts at t₀ and ends att₀+T. Period I extends from t₀ to t₁, period II extends from t₁ to t₂,and period III extends from t₂ to t₀+T. In one embodiment, the currentmagnitude I_(comp) is determined by

$\begin{matrix}{{{Region}\mspace{14mu} I\text{:}\mspace{14mu} I_{comp}} = {{\frac{\beta_{3} \times \left( {t - t_{0}} \right)}{R_{comp}}\mspace{14mu} t_{0}} \leq t \leq t_{1}}} & \left( {{Equation}\mspace{14mu} 43\; A} \right) \\{{{Region}\mspace{14mu}{II}\text{:}\mspace{14mu} I_{comp}} = {{\frac{{\beta_{3} \times \left( {t_{1} - t_{0}} \right)} + {\beta_{4} \times \left( {t - t_{1}} \right)}}{R_{comp}}\mspace{14mu} t_{1}} \leq t \leq t_{2}}} & \left( {{Equation}\mspace{14mu} 43\; B} \right) \\{{{Region}\mspace{14mu}{III}\text{:}\mspace{14mu} I_{comp}} = {{\frac{\begin{matrix}{{\beta_{3} \times \left( {t_{1} - t_{0}} \right)} +} \\{\beta_{4} \times \left( {t_{2} - t_{1}} \right)}\end{matrix}}{R_{comp}}\mspace{14mu} t_{2}} \leq t \leq {t_{0} + T}}} & \left( {{Equation}\mspace{14mu} 43\; C} \right)\end{matrix}$

where β₃ and β₄ are the slopes. For example, as shown in FIG. 15(A),β₃ =S_1w  (Equation 44A)β₄ =S_2w  (Equation 44B)

According to yet another embodiment, a system for protecting a powerconverter includes a threshold generator configured to generate athreshold signal, and a first comparator configured to receive thethreshold signal and a first signal and to generate a comparison signal.The first signal is associated with an input current for a powerconverter. Additionally, the system includes a pulse-width-modulationgenerator configured to receive the comparison signal and generate amodulation signal in response to the comparison signal, and a switchconfigured to receive the modulation signal and adjust the input currentfor the power converter. The threshold signal is associated with athreshold magnitude as a function of time. The threshold magnitudeincreases with time at a first slope during a first period, and thethreshold magnitude increases with time at a second slope during asecond period. The first slope and the second slope are different. Forexample, the system is implemented according to FIG. 17.

According to yet another embodiment, a system for protecting a powerconverter includes a threshold generator configured to generate a firstthreshold signal, and a first comparator configured to receive the firstthreshold signal and a first input signal and to generate a firstcomparison signal. The first input signal is associated with an inputcurrent for a power converter, and the first threshold signal isassociated with a first threshold magnitude as a function of time.Additionally, the system includes a second comparator configured toreceive a second threshold signal and the first input signal and togenerate a second comparison signal. The second threshold signal isassociated with a second threshold magnitude. Moreover, the systemincludes a logic component configured to receive the first comparisonsignal and the second comparison signal and generate an output signal.Also, the system includes a pulse-width-modulation generator configuredto receive the output signal and to generate a modulation signal inresponse to the output signal, and a switch configured to receive themodulation signal and adjust the input current for the power converter.The first threshold magnitude increases with time at a first slopeduring a first period, and the first threshold magnitude increases withtime at a second slope during at least a second period. The first slopeand the second slope are different. For example, the system isimplemented according to FIGS. 17 and/or 18.

According to yet another embodiment, a system for protecting a powerconverter includes a current generator configured to generate a firstcurrent flowing into the current generator, and a comparator configuredto receive a threshold signal and a first signal and to generate acomparison signal. The first signal is a sum of a second signal and athird signal, the second signal is associated with the first current,and the third signal is associated with an input current for a powerconverter. Additionally, the system includes a pulse-width-modulationgenerator configured to receive the comparison signal and generate amodulation signal in response to the comparison signal, and a switchconfigured to receive the modulation signal and adjust the input currentfor the power converter. The first current is associated with a currentmagnitude as a function of time. The current magnitude increases withtime at a first slope during a first period, and the threshold magnitudeincreases with time at a second slope during a second period. The firstslope and the second slope are different. For example, the system isimplemented according to FIG. 19.

According to yet another embodiment, a method for protecting a powerconverter includes generating a threshold signal, and receiving thethreshold signal and a first signal. The first signal is associated withan input current for a power converter. Additionally, the methodincludes processing information associated with the threshold signal andthe first signal, generating a comparison signal based on at leastinformation associated with the threshold signal and the first signal,processing information associated with the comparison signal, generatinga modulation signal based on at least info nation associate with thecomparison signal, and adjusting the input current for the powerconverter in response to the modulation signal. The threshold signal isassociated with a threshold magnitude as a function of time. Thethreshold magnitude increases with time at a first slope during a firstperiod, and the threshold magnitude increases with time at a secondslope during a second period. The first slope and the second slope aredifferent. For example, the method is performed according to FIG. 17.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first threshold signal. The firstthreshold signal is associated with a first threshold magnitude as afunction of time. Additionally, the method includes receiving the firstthreshold signal and a first input signal. The first input signal isassociated with an input current for a power converter. Moreover, themethod includes processing information associated with the firstthreshold signal and the first input signal, generating a firstcomparison signal based on at least information associated with thefirst threshold signal and the first input signal, and receiving asecond threshold signal and the first input signal. The second thresholdsignal is associated with a second threshold magnitude. Also, the methodincludes processing information associated with the second thresholdsignal and the first input signal, generating a second comparison signalbased on at least information associated with the second thresholdsignal and the first input signal, receiving the first comparison signaland the second comparison signal, and generating an output signal basedon at least information associated with the first comparison signal andthe second comparison signal. Additionally, the method includesprocessing information associated with the output signal, generating amodulation signal based on at least information associated with theoutput signal, and adjust the input current for the power converter inresponse to the modulation signal. The first threshold magnitudeincreases with time at a first slope during a first period, the firstthreshold magnitude increases with time at a second slope during atleast a second period, and the first slope and the second slope aredifferent. For example, the method is performed according to FIGS. 17and/or 18.

According to yet another embodiment, a method for protecting a powerconverter includes generating a first current flowing into a currentgenerator, and receiving a threshold signal and a first signal. Thefirst signal is a sum of a second signal and a third signal, the secondsignal is associated with the first current, and the third signal isassociated with an input current for a power converter. Additionally,the method includes processing information associated with the thresholdsignal and the first signal, generating a comparison signal based on atleast information associated with the threshold signal and the firstsignal, processing information associated with the comparison signal,generating a modulation signal based on at least information associatedwith the comparison signal, and adjusting the input current for thepower converter in response to the modulation signal. The first currentis associated with a current magnitude as a function of time. Thecurrent magnitude increases with time at a first slope during a firstperiod, and the threshold magnitude increases with time at a secondslope during a second period. The first slope and the second slope aredifferent. For example, the method is performed according to FIG. 19.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system for protecting a power converter, the system comprising: a voltage generator configured to generate a first voltage signal, the first voltage signal being associated with a first voltage magnitude; a processing component including one or more comparators, the processing component configured to receive the first voltage signal and an input signal and to generate an output signal, the input signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the output signal, generate a modulation signal based on at least information associated with the output signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the processing component is further configured to generate the output signal in a first logic state in response to an input magnitude for the input signal being larger than an effective threshold magnitude, and to generate the output signal in a second logic state in response to the input magnitude being smaller than the effective threshold magnitude; the first logic state and the second logic state are different; the effective threshold magnitude remains constant during a first substantial period of time; the effective threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the effective threshold magnitude remains constant during a third substantial period of time; the processing component is further configured to receive a second voltage signal, the second voltage signal being associated with a second voltage magnitude; and the effective threshold magnitude is equal to the second voltage magnitude during the third substantial period of time.
 2. The system of claim 1 wherein: one of the first logic state and the second logic state is associated with a logic high level; and the other one of the first logic state and the second logic state is associated with a logic low level.
 3. The system of claim 1 wherein the pulse-width-modulation generator is further configured to turn off the switch in response to the output signal being in the first logic state.
 4. The system of claim 1 wherein the effective threshold magnitude is periodic with a threshold period, the threshold period including the first substantial period of time, the second substantial period of time, and the third substantial period of time.
 5. The system of claim 1 wherein: the power converter includes an inductive winding; and the input current is configured to flow through the inductive winding.
 6. The system of claim 1, and further comprising a resistor configured to convert the input current for the power converter to a converted voltage, the converted voltage being the input signal.
 7. The system of claim 1 wherein the effective threshold magnitude is equal to the first voltage magnitude during the second substantial period of time.
 8. The system of claim 7 wherein the effective threshold magnitude is equal to the first voltage magnitude during the first substantial period of time.
 9. A system for protecting a power converter, the system comprising: a voltage generator configured to generate a first voltage signal, the first voltage signal being associated with a first voltage magnitude; a processing component including one or more comparators, the processing component configured to receive the first voltage signal and an input signal and to generate an output signal, the input signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the output signal, generate a modulation signal based on at least information associated with the output signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the processing component is further configured to generate the output signal in a first logic state in response to an input magnitude for the input signal being larger than an effective threshold magnitude, and to generate the output signal in a second logic state in response to the input magnitude being smaller than the effective threshold magnitude; the first logic state and the second logic state are different; the effective threshold magnitude remains constant during a first substantial period of time; the effective threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the effective threshold magnitude remains constant during a third substantial period of time; the processing component includes a resistor configured to receive a first current and generate a second voltage based on at least information associated with the first current; and the effective threshold magnitude is equal in magnitude to a sum of a predetermined threshold magnitude and the second voltage.
 10. A system for protecting a power converter, the system comprising: a voltage generator configured to generate a first voltage signal, the first voltage signal being associated with a first voltage magnitude; a processing component including one or more comparators, the processing component configured to receive the first voltage signal and an input signal and to generate an output signal, the input signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the output signal, generate a modulation signal based on at least information associated with the output signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the processing component is further configured to generate the output signal in a first logic state in response to an input magnitude for the input signal being larger than an effective threshold magnitude, and to generate the output signal in a second logic state in response to the input magnitude being smaller than the effective threshold magnitude; the first logic state and the second logic state are different; the effective threshold magnitude remains constant during a first substantial period of time; the effective threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the effective threshold magnitude remains constant during a third substantial period of time; and the pulse-width-modulation generator is further configured to receive the input signal, generate the modulation signal based on at least information associated with the input signal and the output signal, and output the modulation signal to the switch to adjust the input current for the power converter.
 11. A system for protecting a power converter, the system comprising: a voltage generator configured to generate a first voltage signal, the first voltage signal being associated with a first voltage magnitude; a processing component including one or more comparators, the processing component configured to receive the first voltage signal and an input signal and to generate an output signal, the input signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the output signal, generate a modulation signal based on at least information associated with the output signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the processing component is further configured to generate the output signal in a first logic state in response to an input magnitude for the input signal being larger than an effective threshold magnitude, and to generate the output signal in a second logic state in response to the input magnitude being smaller than the effective threshold magnitude; the first logic state and the second logic state are different; the effective threshold magnitude remains constant during a first substantial period of time; the effective threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the effective threshold magnitude remains constant during a third substantial period of time; and the pulse-width-modulation generator includes: a pulse-width-modulation comparator configured to receive the input signal and generate a comparison signal based on at least information associated with the input signal; a logic controller configured to receive the comparison signal and the output signal and generate a control signal based on at least information associated with the comparison signal and the output signal; and a drive component configured to receive the control signal and generate the modulation signal based on at least information associated with the control signal.
 12. A system for protecting a power converter, the system comprising: a threshold generator configured to generate a threshold signal, the threshold signal being associated with a threshold magnitude; a first comparator configured to receive the threshold signal and a sensing signal and to generate a first comparison signal, the sensing signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the first comparison signal, generate a modulation signal based on at least information associated with the first comparison signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the threshold magnitude remains constant during a first substantial period of time; the threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the threshold magnitude remains constant during a third substantial period of time; and the pulse-width-modulation generator is further configured to receive the sensing signal, generate the modulation signal based on at least information associated with the sensing signal and the first comparison signal, and output the modulation signal to the switch to adjust the input current for the power converter.
 13. The system of claim 12 wherein the threshold magnitude is periodic with a threshold period, the threshold period including the first substantial period of time, the second substantial period of time, and the third substantial period of time.
 14. The system of claim 12 wherein: the power converter includes an inductive winding; and the input current is configured to flow through the inductive winding.
 15. The system of claim 12, and further comprising a first resistor configured to convert the input current for the power converter to a first voltage, the first voltage being the sensing signal.
 16. The system of claim 15 wherein the threshold signal is a threshold voltage.
 17. The system of claim 12 wherein the modulation signal is configured to turn off the switch in response to the comparison signal indicating that a first magnitude for the sensing signal is equal to or larger than the threshold magnitude.
 18. A system for protecting a power converter, the system comprising: a threshold generator configured to generate a threshold signal, the threshold signal being associated with a threshold magnitude; a first comparator configured to receive the threshold signal and a sensing signal and to generate a first comparison signal, the sensing signal being associated with an input current for the power converter; and a pulse-width-modulation generator configured to receive the first comparison signal, generate a modulation signal based on at least information associated with the first comparison signal, and output the modulation signal to a switch to adjust the input current for the power converter; wherein: the threshold magnitude remains constant during a first substantial period of time; the threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the threshold magnitude remains constant during a third substantial period of time; and the pulse-width-modulation generator includes: a second comparator configured to receive the sensing signal and generate a second comparison signal based on at least information associated with the sensing signal; a logic controller configured to receive the second comparison signal and the first comparison signal and generate a control signal based on at least information associated with the first comparison signal and the second comparison signal; and a drive component configured to receive the control signal and generate the modulation signal based on at least information associated with the control signal.
 19. A method for protecting a power converter, the method comprising: generating a first voltage signal, the first voltage signal being associated with a first voltage magnitude; receiving the first voltage signal and an input signal, the input signal being associated with an input current for a power converter; processing information associated with the first voltage signal and the input signal; generating an output signal based on at least information associated with the first voltage signal and the input signal; processing information associated with the output signal; generating a modulation signal based on at least information associated with the output signal; and outputting the modulation signal to adjust the input current for the power converter; wherein the generating an output signal includes: generating the output signal in a first logic state in response to an input magnitude for the input signal being larger than an effective threshold magnitude; and generating the output signal in a second logic state in response to the input magnitude being smaller than the effective threshold magnitude; wherein: the first logic state and the second logic state are different; the effective threshold magnitude remains constant during a first substantial period of time; the effective threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; the effective threshold magnitude remains constant during a third substantial period of time; the receiving the first voltage signal and an input signal includes receiving a second voltage signal, the second voltage signal being associated with a second voltage magnitude; and the effective threshold magnitude is equal to the second voltage magnitude during the third substantial period of time.
 20. The method of claim 19 wherein: one of the first logic state and the second logic state is associated with a logic high level; and the other one of the first logic state and the second logic state is associated with a logic low level.
 21. The method of claim 19 wherein the effective threshold magnitude is equal to the first voltage magnitude during the second substantial period of time.
 22. The method of claim 19 wherein the effective threshold magnitude is equal to the first voltage magnitude during the first substantial period of time.
 23. A method for protecting a power converter, the method comprising: generating a threshold signal, the threshold signal being associated with a threshold magnitude; receiving the threshold signal and a sensing signal, the sensing signal being associated with an input current for a power converter; processing information associated with the threshold signal and the sensing signal; generating a comparison signal based on at least information associated with the threshold signal and the sensing signal; processing information associated with the comparison signal; generating a modulation signal based on at least information associate with the comparison signal; and outputting the modulation signal to adjust the input current for the power converter; wherein: the threshold magnitude remains constant during a first substantial period of time; the threshold magnitude changes linearly with time at a slope during a second substantial period of time, the slope being not equal to zero; and the threshold magnitude remains constant during a third substantial period of time; wherein the generating a modulation signal based on at least information associate with the comparison signal includes: receiving the sensing signal; and generating the modulation signal based on at least information associated with the sensing signal and the comparison signal.
 24. The method of claim 23 wherein the threshold magnitude is periodic with a threshold period, the threshold period including the first substantial period of time, the second substantial period of time, and the third substantial period of time. 